Studies of the cosmic microwave background (CMB) radiation have driven the current era of precision cosmology. The
tightest cosmological constraints to date have been derived from the primary CMB anisotropies, which predominantly
probe the universe in its infancy. However, CMB experiments have recently entered a new regime in which constraints
derived from the secondary anisotropies -- sourced by effects between our vantage point and the surface of last
scattering -- substantially improve upon those derived from the primary anisotropies alone. Moreover, the secondary
anisotropies contain valuable astrophysical information about the distribution of baryons and dark matter at late times.
I will describe new approaches to extract information from these signals, focusing in particular on the kinematic
Sunyaev-Zel'dovich effect, which refers to the Compton-scattering of CMB photons off free electrons with non-zero bulk
momentum along the line-of-sight. I will show how I have used this effect to probe the abundance of ionized gas in and
around modern-day galaxies and thereby resolve the long-standing "missing baryon problem". I will conclude with a
look ahead to such measurements with the Advanced Atacama Cosmology Telescope and Simons Observatory, which
will measure CMB secondary anisotropies -- and search for primordial gravitational waves -- with unprecedented
sensitivity.